The invention relates to a method of manufacturing an optical cable element. The method is a continuous process in which one or more optical fibers are provided around a central draw core. The fibers are either provided in slots in the surface of a synthetic resin sheath extruded around the draw core, or in synthetic resin tubes which loosely envelop the fibers and which are connected to the surface of the draw core with an adhesive. The slots or the tubes follow helical paths whose directions of rotation vary periodically.
The method is used to manufacture two types of optical cable elements. The first type is the so-called slotted-core-type, and the second type is the so-called loose-tube-type. Both types of cable elements known.
A slot or a synthetic resin tube as a rule contains only one optical fiber. However, it is possible to provide more fibers per slot or tube. The synthetic resin sheath extruded around the core as a rule has several slots extending parallel to each other. Analogously in an optical cable element of the loose-tube-type, several mutually parallel synthetic resin tubes are as a rule provided around the core. It is even possible to provide several layers of synthetic resin tubes around the core.
A helical path with a varying direction of rotation is also known as an SZ-configuration. It is very difficult to give the slots or the tubes the SZ configuration. In tubes, for example, an assembly of storage reels is rotated around the draw core with periodically varying direction of rotation. This requires the use of complicated and expensive apparatus.
Published Japanese patent application Kokai No. 52-126238 describes a method of manufacturing a synthetic resin core with SZ slots by using an extruder which rotates periodically to the left and to the right. A rotating extruder makes the process expensive and more complicated. The greatest disadvantage is that the extrusion rate is reduced. Due to shearing effects, the slots are crushed or are otherwise deformed if the extrusion rate is not reduced in this method.
In the process described in U.S. Pat. No. 4,205,899 a stationary (i.e. non-rotating) extruder is used. The nozzle of the extruder has such a shape that slots are formed in the surface of the extruded sheath. The sheath is cooled in such manner that it has reasonably solidified. The sheath is then twisted by using a twister which has a disc which is rotatable about the sheath. The disc has blades (reference numeral 19 in FIG. 4) which engage in the slots. By rotating the disc alternately to the left and to the right, shearing forces are exerted on the sheath which result in the slots, at the location where the synthetic resin leaves the nozzle and where the synthetic resin is most plastic, obtains an SZ configuration. The sheath is then further cooled and the slots are provided with optical fibers.
The disadvantage of the process described in U.S. Pat. No. 4,205,999 is that the core is cooled in two phases, and hence an interrupted cooling duct must be used. A more serious disadvantage is that the process is very critical due to the shearing forces which must be transmitted to the area where the synthetic resin leaves the nozzle. The danger exists that either the groove does not obtain the desired shape or the groove is damaged.